Q8 - Fluoride enters a region's groundwater when rain

[mshinners]

Question Type:
Inference (Most Strongly Supported)

Stimulus Breakdown:
When rain dissolves F-rocks, it gets into the groundwater. When a bunch of stuff is held constant, F is higher when there's S.

Answer Anticipation:
Seems like S has something to do with F concentrations. Since we know F gets into the groundwater when F-rocks dissolve, the inference here connects S with F dissolving.

Correct answer:
(D)

Answer choice analysis:
(A) Out of scope. There's no information about other sources of F.

(B) Since the information we're given about rainfall is that it was held constant, we can't infer what, if any, effect it has. That said, if anything, this is contradicted since we know rainfall is a relevant variable.

(C) Unwarranted comparison. While we can infer that S affects F dissolution, there's no information comparing the rates between those two elements.

(D) Bingo. This answer combines our two premises. I wouldn't pick it on the first go (it doesn't have to be true, but, then again, the question isn't asking for that), but I'd definitely leave it and decide on it after reading (E).

(E) Relative vs. absolute. The stimulus mentions "significantly higher" concentrations of fluoride. However, that doesn't guarantee the concentrations are "high", as this answer states. Tricky! Also, the argument specifically states concentrations of F-rocks were held constant.

Takeaway/Pattern:
Watch out for relative language in the stimulus. If it's there, it's very likely there will be a tricky trap answer that jumps to absolute language.

#officialexplanation

[wowpersonage1]

Hey Geeks,

I need your help please !

I was down between answer D and E, however, ruled out answer D because the rate at which fluoride-bearing minerals dissolves seems out of scope. I was able to rule all the other choices and was left with E which seems to make some sense that if there is high concentration of sodium bearing minerals then it contains high concentration of fluoride bearing minerals which explains why there are significantly higher fluoride concentrations which also contained high concentration of sodium.

[pewals13]

Hey

I think you overlooked the line that states "rainfall, concentrations of fluoride bearing minerals, and other relevant variables are held constant"

Basically imagine that you have two identical groundwater scenarios in a study where the only difference between them is the concentration of sodium. The study found that there's more fluoride in the groundwater of the scenario with the sodium.

You are looking for an answer choice that explains the differing result. You can expect it will deal with sodium causing the increased fluoride concentration since this is the only difference between the two scenarios in the study (we are explicitly told that all other relevant variables are held constant).

Answer choices:

(A) We don't know what the primary source of fluoride in the groundwater is, we only know that when sodium is present, all other things being equal, it correlates with higher concentrations of fluoride

(B) This seems to directly contradicted by the stimulus

(C) There is no information provided concerning the speed of dissolution of fluoride-bearing minerals or sodium-bearing minerals. I can't see how this could be concluded from the fact that high sodium concentrations are correlated with high fluoride concentrations.

(D) This is supported by the evidence because it provides a potential explanation for the correlation between fluoride and sodium levels.

It is known from the stimulus that "when rain dissolves fluoride-bearing minerals in the soil------>fluoride enters the region's groundwater"

So if sodium increases the rate at which fluoride is added to the groundwater (dissolved) the result is a higher concentration of fluoride during a given unit of rain.

(E) Makes perfect sense. But wait! The stimulus says this was held constant in the study. Therefore, this cannot account for the difference between the two scenarios.

[Diane.S]

pewals13 thank you for your reply.

I'm a little confused about the question type of this question. "...most reasonably be conclude on the basis of..." Wouldn't this wording indicate inference question?

I had thought that this was an inference question but the correct answer choice seems to require a bit more than inference. To me, it seems more like a paradox question/strengthen question in which it requires external information to explain the researcher's findings that when fluoride concentrations are higher, sodium concentration is high also.

Thanks in advance!

[kyuya]

I think the two difficult answer choices to differentiate here are (D) and (E). Above provides a good explanation for these but I thought I'd provide additional explaining for those (like me) who though these two were really tough to distinguish.

Firstly, the stimulus tells us a few things:

- Fluoride goes into a regions ground water when it rains and dissolves things that have fluoride in them. Makes sense.
- But -- when we control for variables such as the amount of rain, and amount of things that have fluoride in them we find that there is more fluoride in groundwater that also has high amounts of sodium.

So, to be clear, lets introduce two hypothetical scenarios:

Scene one: It rains, 4 inches and there is 5 mg of sodium dissolving things. We test the ground water, and we find 2 mg of fluoride has been dissolved. We have 3 mg of sodium in the groundwater.

Scene two: It rains 4 inches and there is 5 mg of sodium dissolving things. We test the ground water, and we find 4 mg of fluoride. We have 5 mg of sodium in the groundwater.

What is different in these scenarios? Well the first sentence in both is the exact same, because they are held constant like we are told in the stimulus. However, the concentration of fluoride in the groundwater varies from 4-2 from scenes one and two respectively. Hmm thats strange, why would that be? The first variables (in the first sentence) are the exact same.

Ah! But if we go to the final sentence, we find that less sodium ultimately correlates with less fluoride concentration in the ground water. Or, in other words, the more sodium we have, the higher the concentration.This is a correlation. What can explain this relationship?

Well, (D) provides us with an answer choice that does. It says that sodium allows for fluoride to dissolve faster, and therefore would have a higher concentration. Look at the two scenes above.. the higher sodium does seem to correlate to more fluoride being dissolved, doesn't it? This answer choice explains it and we can therefore reasonably conclude it is true.

(E) Firstly, the word soil may be problematic. Remember we are talking about ground water. However, these two things could be related, but its certainly an assumption, while (D) requires no such logical jump.

Furthermore, fluoride concentrations are held constant in the stimulus so this answer choice ends up being irrelevant. We want to know why, in spite of the fact that these variables are held constant, this correlation occurs.

The tough thing about this stimulus is teasing out that they want you to explain or conclude something about a relatively simple correlation. Fluoride is higher where sodium is higher. Therefore, one of these variables can reasonably be assumed to effect each other (since other factors are held constant) and the AC happens to go with the idea that sodium influences fluoride levels.

[BaopuZ34]

Hey,
I did choose D, but found it a little problematic.
The stimulus mentions "concentration of F-Bearing minerals" and "concentration of F", so I felt it is a two-phase dissolution.

First, the F-beating mineral goes into water, and then it continues to dissolve from the F-mineral to F. Otherwise, it is hard to understand what is concentration of F-minerals in water.

As a result, if D is correct that the sodium increases the rate of F-mineral's dissolution into F. How can concentration of F-minerals "hold constant"? The only reasonable inference seems to be that the sodium also accelerates the rate of the first phase - it increases the rate at which F-mineral goes into water.
However, answer D steps aside and gives no response to this problem...

[Chi YongP398]

Hello,

I was deciding between A and D but ended up choosing A.

Because in answer A, I thought of the possibility that sodium could be the source of fluoride, and therefore fluoride-bearing minerals were actually not the only/primary source of fluoride. Would this reasoning be completely false? I think if I was aware that sodium is an element that could not bear any other elements, I wouldn't have assumed this. For someone who is completely ignorant in these types of subjects, is this something that would be hard to overcome?

I also eliminated D because I thought increasing the rate of dissolution seemed a bit too broad and therefore irrelevant. In the stimulus, we are told that "relevant variables are held constant", would this include length of time from which they started the experiment to which they measured the concentration level of fluoride? Because I would assume that if groundwater with high concentration of sodium was measured way later than the other, the former would just have more time to accumulate fluoride than having the fluoride bearing minerals to dissolve at a greater speed.